This invention is generally in the field of devices for the transport of therapeutic or biological molecules across tissue barriers, such as for drug delivery or sampling of biological fluids.
Numerous drugs and therapeutic agents have been developed in the battle against disease and illness. However, a frequent limitation of these drugs is their delivery: how to transport drugs across biological barriers in the body (e.g., the skin, the oral mucosa, the blood-brain barrier), which normally do not transport drugs at rates that are therapeutically useful or optimal.
Drugs are commonly administered orally as pills or capsules. However, many drugs cannot be effectively delivered in this manner, due to degradation in the gastrointestinal tract and/or elimination by the liver. Moreover, some drugs cannot effectively diffuse across the intestinal mucosa. Patient compliance may also be a problem, for example, in therapies requiring that pills be taken at particular intervals over a prolonged time.
Another common technique for delivering drugs across a biological barrier is the use of a needle, such as those used with standard syringes or catheters, to transport drugs across (through) the skin. While effective for this purpose, needles generally cause pain; local damage to the skin at the site of insertion; bleeding, which increases the risk of disease transmission; and a wound sufficiently large to be a site of infection. The withdrawal of bodily fluids, such as for diagnostic purposes, using a conventional needle has these same disadvantages. Needle techniques also generally require administration by one trained in its use. The needle technique also is undesirable for long term, controlled continuous drug delivery.
Similarly, current methods of sampling biological fluids are invasive and suffer from the same disadvantages. For example, needles are not preferred for frequent routine use, such as sampling of a diabetic""s blood glucose or delivery of insulin, due to the vascular damage caused by repeated punctures. No alternative methodologies are currently in use. Proposed alternatives to the needle require the use of lasers or heat to create a hole in the skin, which is inconvenient, expensive, or undesirable for repeated use.
An alternative delivery technique is the transdermal patch, which usually relies on diffusion of the drug across the skin. However, this method is not useful for many drugs, due to the poor permeability (i.e. effective barrier properties) of the skin. The rate of diffusion depends in part on the size and hydrophilicity of the drug molecules and the concentration gradient across the stratum corneum. Few drugs have the necessary physiochemical properties to be effectively delivered through the skin by passive diffusion. Iontophoresis, electroporation, ultrasound, and heat (so-called active systems) have been used in an attempt to improve the rate of delivery. While providing varying degrees of enhancement, these techniques are not suitable for all types of drugs, failing to provide the desired level of delivery. In some cases, they are also painful and inconvenient or impractical for continuous controlled drug delivery over a period of hours or days. Attempts have been made to design alternative devices for active transfer of drugs, or analyte to be measured, through the skin.
For example, U.S. Pat. No. 5,879,326 to Godshall et al. and PCT WO 96/37256 by Silicon Microdevices, Inc. disclose a transdermal drug delivery apparatus that includes a cutter portion having a plurality of microprotrusions, which have straight sidewalls, extending from a substrate that is in communication with a drug reservoir. In operation, the microprotrusions penetrate the skin until limited by a stop region of the substrate and then are moved parallel to the skin to create incisions. Channels in the substrate adjacent to the microprotrusions allow drug from the reservoir to flow to the skin near the area disrupted by the microprotrusions. Merely creating a wound, rather than using a needle which conveys drug through an enclosed channel into the site of administration, creates variability in dosage.
U.S. Pat. No. 5,250,023 to Lee et al. discloses a transdermal drug delivery device, which includes a plurality of needles having a diameter in the range of 50 to 400 xcexcm. The needles are supported in a water-swellable polymer substrate through which a drug solution permeates to contact the surface of the skin. An electric current is applied to the device to open the pathways created by the needles, following their withdrawal from the skin upon swelling of the polymer substrate.
PCT WO 93/17754 by Gross et al discloses another transdermal drug delivery device that includes a housing having a liquid drug reservoir and a plurality of tubular elements for transporting liquid drug into the skin. The tubular elements may be in the form of hollow needles having inner diameters of less than 1 mm and an outer diameter of 1.0 mm.
While each of these devices has potential use, there remains a need for better drug delivery devices, which make smaller incisions, deliver drug with greater efficiency (greater drug delivery per quantity applied) and less variability of drug administration, and/or are easier to use. In view of these needs, microneedle devices have been developed, which are described in U.S. Ser. Nos. 09/095,221, filed Jun. 10, 1998, and 09/316,229, filed May 21, 1999, both by Prausnitz et al., which are hereby incorporated by reference. Certain embodiments of the device were found to readily penetrate skin samples in in vitro experiments, but not always provide uniform or complete insertion of the microneedles into some areas of the skin in vivo, as the stratum corneum and underlying tissues are highly deformable and elastic over much of the body.
It is therefore an object of the present invention to provide methods and devices for improving the control of microneedle insertion into the body of a patient.
It is another object of the present invention to provide a microneedle device producing improved microneedle insertion for relatively painless, controlled, safe, convenient transdermal delivery of drugs.
It is a further object of the present invention to provide a microneedle device producing improved microneedle insertion for controlled sampling of biological fluids in a minimally-invasive, painless, and convenient manner.
Microneedle devices and methods of use thereof are provided for the enhanced transport of molecules, including drugs and biological molecules, across tissue by improving the interaction of an array of microneedles and a deformable, elastic biological barrier, such as human skin. The devices and methods act to (1) limit the elasticity, (2) adapt to the elasticity, (3) utilize alternate ways of creating the holes for the microneedles to penetrate the biological barrier, other than the simply direct pressure of the microneedle substrate to the barrier surface, or (4) any combination of these methods.
In preferred embodiments for limiting the elasticity of skin, the microneedle device includes features suitable for stretching, pulling, or pinching the skin to present a more rigid, less deformable, surface in the area to which the microneedles are applied (i.e. penetrate). For example, a vacuum can be applied to the area of the skin at the site of microneedle application to pull it taut and/or pull the skin onto the tips of the microneedles. Alternatively or in addition, the elasticity of skin can be reduced by applying a thin film or membrane over the skin surface at the site of application, so as to keep the skin tight, limiting the ability of the skin to stretch at the application site. The microneedles are then pushed through the film or membrane and into the skin.
In preferred embodiments for adapting the device to the elasticity of skin, the microneedles of the device include individual extensions or are provided in a curved three dimensional array, for example, by using a flexible substrate and/or varying the height of the microneedles in the array. In another embodiment, the microneedles are applied to the skin surface at an increased velocity, thereby reducing the time available for the stratum corneum and underlying tissues to deform from contact with the tips or entire length of the microneedles.
In a preferred embodiment for creating holes in the skin for microneedles, tiny holes are burned into the skin, for example, by heating of the tips of the microneedles and/or by using a laser. In another embodiment, a focused blast of high pressure gas is used to create the holes.
Essentially all of the microneedle devices and methods described herein can be adapted to vibrate the microneedles and/or the skin to further enhance penetration. In a preferred embodiment, the microneedles include a lubricating material coated onto or incorporated into the microneedles.